#2408: Cactus build output is very verbose

Comment (by Roland Haas):

Oh never mind. Make is actually adding the “+=” option at the beginning. Probably same detail of how recursive variables work https://www.gnu.org/software/make/manual/html_node/Appending.html . I have applied Erik’s fix as git hash d14d52bf "Cactus: propagate --silent option sub-makes" of cactus